PSC motor system for use in HVAC applications with field adjustment and fail-safe capabilities

ABSTRACT

A field-adjustable motor control system including a motor, an inverter coupled to provide energization to the motor, a controller coupled to the inverter that provides signals to control the output of the inverter in response to received input control signals and in response to field adjustment signals from a first field adjustment circuit. In certain embodiments, fail-safe circuitry is also provided.

BACKGROUND OF THE INVENTION

[0001] The present disclosure relates to motor control systems and, moreparticularly, to permanent split capacitor (“PSC”) motor control systemsfor use in heating, ventilation, and air conditioning (“HVAC”)applications.

[0002] Conventional HVAC applications often utilize multi-tapped PSCtype motors. In general, a multi-tapped PSC motor is a motor that has amulti-tapped main winding where all or part of the main winding iscoupled in parallel with an auxiliary starting winding that is coupledin series with a capacitor. Such multi-tapped PSC motors are used inHVAC applications, such as furnace blower and air handler applications,because the multi-tapped winding can produce variable output torque and,therefore, variable output speed for the purpose of delivering differentamounts of air flow for different applications. For example, one tapsetting may be provided to provide a relatively low amount of air flowto provide for air circulation when there is no heating or coolingactivity. Another tap setting could be provided to increase the air flowwhen cooling is desired. By using multiple taps, various operatingstates can be established for a tapped PSC motor, such as heating,cooling, and air. In general, each tap point on the multi-tapped PSCmotor is coupled to an input line and relays are energized in responseto control signals from, for example, a thermostat to provideenergization to one of the tap points at any given time.

[0003] One characteristic of multi-tapped PSC motors when used with airblowers, such as a squirrel cage blower, is that the Speed vs. Torquecurves for such systems are not constant, but have a generally “reverseC shape” wherein the torque will increase with speed up to a maximumpoint but, thereafter, as the speed increases the torque will begin todecrease. FIG. 1 generally illustrates the Speed vs. Torquecharacteristics for a conventional multi-tapped PSC motor for low,medium, medium high and high settings with each setting having its ownSpeed vs. Torque curve. As the figure illustrates, for each Speed vs.Torque curve, as speed increases the output torque will initiallyincrease from a minimum value at or near zero speed to a maximum valueand then decrease to near or zero torque at a maximum speed.

[0004] In addition to having non-linear Speed vs. Torquecharacteristics, the operation of conventional multi-tapped PSC motorscan be significantly impacted by the static pressure of the environmentin which the system is operating. This is reflected by FIGS. 1 and 2,where FIG. 1 was described above, and FIG. 2 illustrates Static Pressure(in inches of water) vs. Air flow (in cubic feet per minute (CFM)) forthe various taps of a conventional multi-tapped PSC motor. Linesreflecting average, low and high static pressures are illustrated inFIGS. 1 and 2.

[0005] As will be appreciated from FIGS. 1 and 2, for a given tapsetting, as the static pressure is increased above the average staticpressure value, the speed of the motor will increase. This speedincrease will, therefore, result in a decrease in the output torque ofthe blower and accordingly a decrease in the output airflow from theblower. The reverse may occur if the static pressure drops below theaverage value. Because of this influence of the static pressure on theoutput airflow, in most HVAC systems using a multi-tapped PSC motor, theoperation of the system will vary (perhaps significantly) from day today, month to month as the static pressure within which the systemoperates changes. Such variations provide for unstable and inconsistentoperation which is undesirable.

[0006] The present disclosure describes several embodiments a motorcontrol system for a PSC motor that are designed to address thedescribed and other limiting characteristics to conventional systems toprovide an improved motor control system.

SUMMARY OF THE INVENTION

[0007] In accordance with one exemplary embodiment constructed inaccordance with certain teachings of the present disclosure, a motorcontrol system is provided that includes a motor, an inverter coupled toprovide energization to the motor, a controller coupled to the inverter,the controller providing signals to control the output of the inverterin response to received input control signals to provide a level ofoutput power to the motor, the desired level of output power beingdetermined at least in part by the input control signals.

[0008] In accordance with another exemplary embodiment constructed inaccordance with certain teachings of the present disclosure, a fail-safemotor control system is provided that includes a motor, an invertercoupled to provide energization to the motor, a controller coupled tothe inverter, the controller providing signals to control the output ofthe inverter in response to received input control signals and inresponse to field adjustment signals, a relay coupled at its inputs to asource of line power and to the output of the inverter and at its outputto the motor, the relay receiving control signals from the controllerand being configured such that, in the event of a failure of thecontroller, the relay will couple the line power to the motor.

[0009] In accordance with yet a further exemplary embodiment constructedin accordance with certain teachings of the present disclosure afield-adjustable motor control system is provided comprising a motor; aninverter coupled to provide energization to the motor; a controllercoupled to the inverter, the controller providing signals to control theoutput of the inverter in response to received input control signals andin response to field adjustment signals, wherein the input controlsignals can define a first and a second operating state of thecontroller, each of the first and second operating states correspondingto a desired operating state of the system; a first field adjustmentcircuit for providing a first field adjustment signal to the controller,the first field adjustment signal, in combination with the input controlsignals, defining a desired output parameter of the inverter for thefirst operating state; and a relay coupled at its inputs to a source ofline power and to the output of the inverter and at its output to themotor, the relay receiving control signals from the controller and beingconfigured such that, in the event of a failure of the controller, therelay will couple the line power to the motor.

[0010] Other aspects of the present disclosure will be apparent from areview of the disclosure, the figures and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The description is presented with reference to the accompanyingdrawings in which:

[0012]FIG. 1 generally illustrates the Speed vs. Torque characteristicsfor a conventional multi-tapped PSC motor for low, medium, medium highand high settings with each setting having its own Speed vs. Torquecurve.

[0013]FIG. 2 illustrates Static Pressure (in inches of water) vs. Airflow (in cubic feet per minute (CFM)) for the various taps of aconventional multi-tapped PSC motor as illustrated in FIG. 1.

[0014]FIGS. 3A and 3B generally illustrate an exemplary permanent splitcapacitor (“PSC”) induction motor control system constructed inaccordance with certain teachings of this disclosure for use, forexample, as a blower drive for an HVAC application

[0015]FIG. 4 generally illustrates an exemplary non-tapped (singlespeed) PSC motor including a main winding that is coupled in parallelwith a series connection of an auxiliary winding and a capacitor for usein the exemplary system of FIGS. 3A and 3B.

[0016]FIG. 5 generally illustrates an exemplary embodiment of the inputconversion circuitry 8 of FIGS. 3A and 3B for converting relatively highvoltage level signals 10 (e.g., 24V or 115V signals) in one format intologic level signals (e.g., 5V signals) of another format for use indetermining the operating state of the control system.

[0017]FIG. 6 generally illustrates an exemplary lower leg currentmonitoring circuit for monitoring the current in the lower leg of theinverter 4 of FIGS. 3A and 3B.

[0018]FIG. 7 generally illustrates representative volts/hertz curves forsix exemplary operating states of the exemplary control system of FIGS.3A and 3B.

[0019]FIG. 8 generally compares exemplary CFM/Static Pressure curves forone exemplary embodiment of the system of FIGS. 3A and 3B operating inthe CURRENT CONTROL MODE with exemplary curves for a conventionalmulti-tapped PSC motor.

[0020] FIGS. 9A-9C illustrate in greater detail one exemplaryconfiguration of the field adjustment circuits of FIGS. 3A and 3B.

[0021]FIG. 10 illustrates an exemplary control scheme that may beimplemented by the controller 18 of FIGS. 3A and 3B.

[0022]FIG. 11 generally illustrates an exemplary during a start-upoperation that may be implemented by the controller 18 of FIGS. 3A and3B where the PSC motor 2 goes from an off state to a running state, andwhere the voltage and frequency output of the inverter 4 are controlledfrom a predetermined frequency to provide optimum starting of the motor.

[0023]FIG. 12 generally illustrates an alternate exemplary start-upprocess in which the frequency output of the inverter is driven to aboveline frequency during start-up.

[0024]FIG. 13 generally illustrates an exemplary embodiment of thesystem of FIGS. 3A and 3B wherein the relay 6 is configured such thatthe relay, in its unenergized state, couples the PSC motor 2 to theinverter 4.

[0025]FIG. 14 generally illustrates an exemplary embodiment of an uppercurrent trip circuit than may be used with the system of FIGS. 3A and 3Bto monitor the current in the upper leg of the inverter.

[0026]FIG. 15 generally illustrates alternate embodiments of the systemof FIGS. 3A and 3B where the inverter operates off of a single DC bussobtained by full wave rectifying the input line voltage.

[0027] FIGS. 16A-16C generally illustrate an exemplary mountingstructure that may be used with the control system of FIGS. 3A and 3Bwherein a control module of the system 1 (which includes all majorcomponents of the system except for the motor) is mounted within abracket like device that may be readily secured to a blower enclosure.

DESCRIPTION OF EMBODIMENTS

[0028] Turning to the drawings and in particular to FIGS. 3A and 3B, apermanent split capacitor (“PSC”) induction motor control system 1 foruse, for example, as a blower drive for an HVAC application isillustrated.

[0029] The illustrated PSC inverter system 1 comprises six primarycomponents and/or componant systems: (i) a permant split capacitor motor2; (ii) a variable frequency inverter 4 coupled to provide ouput powerto the motor 2; (iii) a switching relay 6, configured to couple theinput of the PSC motor 2 to either the output of the variable frequencyinverter 4 or line power; (iv) an input converter 8 that receives inputsignals 10 in one form from, for example, a furnace board or athermostat and converts the same to logic level control signals 12; (v)a field adjustment system 14 that can be set in the field to providevariable field adjustment signals 16 to adjust the effect of the controlsignals 12 on the operation of the motor control system; and (vi) acontrol circuit 18 that receives control signals 12 and field adjustmentsignals 16 and controls the variable frequency inverter 4 and theswitching relay 6 to drive the motor 2 in a desired manner. Thecomponants of the system are illustrated in a block form in FIG. 3A andin more detail in FIG. 3B.

[0030] In general, AC line power is provided as an input to the variablefrequency inverter 4. The variable freqency inverter 4 converts the ACline power to a DC bus voltage and then converts the DC bus voltage to asingle-phase synthesized sinusoidal waveform of variable voltage andfrequency for application to the motor. AC line power is also providedto a first input contact point for the relay 6 which, in the illustratedembodiment of FIGS. 3A and 3B, is a single pole, double throw relay. Theoutput from the variable frequency inverter 4 is provided to a secondinput for the relay 6. The output of the relay 6 is coupled to one inputterminal of the PSC motor 2. In the example above, the other input tothe PSC motor 2 is coupled to one of the ac input lines.

[0031] In operation, the controller 18 controls the relay 6 to couplethe input of the PSC motor 2 to either the output of inverter 4 or tothe line power. In general, the controller 18 determines the operatingstate of the system in response to the control signals 12 and the fieldadjustment signals 16. Depending on the operating state defined by thecontrol signals provided to the controller 18, the controller willeither: (a) generate signals to switch the relay 6 to couple the motor 2to AC line power, thus operating the motor 2 at a substantially fixedspeed corresponding to the line frequency; or (b) generate signals toswitch the relay 6 to couple the motor 2 to the output of the inverter 4and also generate control signals to control the inverter 4 to provide asingle phase output voltage having appropriate voltage and frequencycharacteristics to drive the PSC motor 2 in a desired manner.

[0032] In many applications of the illustrated system the inverter 4will be driving the motor 2 when the HVAC system is performing activeheating and/or cooling operations. In such applications, the controller18 may be configured to operate in multiple operating states for eachoperation state. For example, the controller 18 may be configured toprovide differing output currents at different settings or differingoutput frequencies or to control the power output of the blower toprovide different CFM outputs. These precise configuration and settingsfor the controller 18 may be software and/or field programmable. As aresult, the installer of a product containing the illustrated PSCInverter System may adjust the operation of the system depending on thecomfort level of the consumer.

[0033] Controller 18 may be a microprocessor-based, software drivencontroller that receives input commands and generates switching signalsfor the relay 6 and the inverter 4 to control the motor 2 in anoptimized manner. In general, the controller 18 controls the relay 6based on the desired output frequency of the inverter. At desired outputfrequencies around line frequency, the controller will typically switchthe relay 6 to couple the PSC motor 2 to the line. The precise speedthreshold level at which such a switching of the relay 6 occurs mayvary. This variation may depend on the specific mode in which thecontroller is operating or whether the motor is going from a higherspeed to a lower speed or vice versa.

[0034] At desired output frequencies below the threshold level, thecontroller 18 will switch the relay 6 to couple the PSC motor 2 to theoutput of the inverter 4. The controller will also generate signals tocontrol the switching of the power switches in the inverter 4 to providean output having the desired voltage and frequency to achieve thedesired output speed. Again, the threshold level where the controllerswitches to the inverter 4 output can be fixed or can vary with theoperating mode of the controller or other conditions.

[0035] In one embodiment, the controller 18 will switch the relay 6 todrive the motor 2 from the inverter 4 when the frequency of the voltageto be applied to the motor 2 is below some fixed percentage of the linefrequency (e.g. 80%, 90% 95% or some other percentage). In thatembodiment, the controller 18 will switch the relay 6 to provide linevoltage when the frequency of the voltage to be applied to the motor isover the fixed percentage. Still further embodiments are envisionedwhere the frequency selected for a line to inverter transition isdifferent from the frequency required for an inverter to linetransition.

[0036] Further, details and alternate constructions of the variouscomponents of the system of FIGS. 3A and 3B are provided below.

[0037] In the example of FIGS. 3A and 3B, the PSC motor 2 is a singlephase PSC motor of a size that is commonly found in circulation blowersfor HVAC applications (e.g., an approximately ⅓ hp to 1 hp). The PSCmotor 2 may be a conventional multi-tapped PSC motor or may be aspecially constructed, non-tapped PSC motor having only two motor inputleads. Generally, a run capacitor as used on conventional PSC motorsshould be used since the embodiments described herein utilize signalphase power to drive the motor. If a multi-tap PSC motor is used, onlythe highest speed tap should typically be used in connection with theillustrated system.

[0038] In a preferred embodiment, a non-tapped (single speed) PSC motor2 is provided that includes a main winding that is coupled in parallelwith a series connection of an auxiliary winding and a capacitor. Aschematic representation of such a motor is illustrated in FIG. 4 wherea main winding 20 is coupled in parallel with a series connection of aauxiliary winding 22 and a capacitor 24.

[0039] To control the amount of noise and/or vibration produced byoperation of the motor 2, it may be desirable to select the windingpattern for the windings 20 and 22 to produce the lowest average noiseover the range of expected operating frequencies. Alternately, inembodiments where it is anticipated that the motor 2 will be operatingpredominately in response to an excitation signal of a given frequency(e.g., a frequency corresponding to an operating state of the systemwhere the blower is in a FAN or CIRCULATION mode), it may be desirableto wind the motor so that the noise/vibration produced at theanticipated predominate operating frequency is minimized. Additionally,in a motor specifically constructed for use with the motor controlsystem described herein, the amount of copper in the main winding can beincreased to increase the overall efficiency of the system.

[0040] While the exemplary system of FIGS. 3A-3B may be used with PSCmotors accross a large range of horsepower ratings, it is anticipatedthat the described systems will be used with PSC motors having ratingsof between ⅕ to 1 Hp.

[0041] As illustrated in FIGS. 3A-3B, the motor control system 1receives input command signals 10 that determine the operating state ofthe system 1. In the illustrated embodiment, the operating state of thesystem 1 is determined based on three logic level control signals 12that are developed and provided by an input converter circuit 8 based onup to five relatively high voltage level signals 10. The relatively highvoltage signals 10 may be provided by, for example, a conventionalthermostat or an ignition control board in a furnace that was designedto control a PSC motor having a multi-tapped winding. The use of theinput converter circuit 8 allows for the motor control system 1 to beused in retrofit applications where the control system 1 will replace aconventional system that operates in response to signals from aconventional thermostat or from control signals provided by an ignitioncontrol board in a furnace.

[0042] Certain existing HVAC systems operate in response to voltagesignals provided by a conventional, e.g., wall-mounted, thermostat. Ingeneral, such conventional thermostats provide output control signals ata level of approximately 24 Volts AC. Although the precise nature of thesignals provided by such conventional thermostats will vary fromthermostat to thermostat, there is typically an output signal “FAN,”that is energized with 24VAC when the fan is to operate in a circulatemode; a “HEAT” output that is energized with 24VAC when the thermostatis indicating that the system is to operate in a heating mode; a “COOL”signal that is energized with 24VAC when the thermostat is indicatingthat the system needs to operate in a cooling mode. Certain types ofthermostats also have a HIGH HEAT and a HIGH COOL signals. The precisemanner in which the 24VAC signals described above are provided by athermostat will vary from thermostat to thermostat. For somethermostats, only one of the output signals (e.g., HEAT) will be activehigh at any given time. For other thermostats, multiple signals may besimultaneously active high (e.g., FAN and HEAT). As described below, theconstruction of the input interface circuit is such that the system canproperly function with a wide variety of thermostats and thermostatsignals.

[0043] In most furnaces a furnace control board or an ignition controlboard uses these 24VAC signals to control various relays located on theignition board. These relays are typically switched to provide 115VACoutput power that is applied directly to one tap of a multi-tap motor.In such systems, only a single output is typically active in a givenoperating mode, as that will be the output used to power the motorcoupled to the HVAC blower at the desired speed. Such ignition controlboard systems typically are capable of providing from three to fivedifferent outputs, with the outputs generally corresponding to FAN(Circulate); HEAT; HIGH HEAT; COOL and HIGH COOL. As described below,the input interface 8 is constructed to be able to properly process such115VAC signal outputs as input commands. The input interface 8 can alsoproperly interpret the 24VAC input signals with some component valuechanges that will be apparent to those of ordinary skill having thebenefit of this disclosure.

[0044]FIG. 5 generally illustrates an exemplary embodiment of the inputconversion circuitry 8 for converting relatively high voltage levelsignals 10 (e.g., 24V or 115V signals) in one format into logic levelsignals (e.g., 5V signals) of another format for use in determining theoperating state of the control system 1.

[0045] Referring to FIG. 5, the input conversion circuitry 8 includes aninterface board which directly receives the signals 10A-10E from eithera thermostat (24V) or a furnace ignition control board (15 VAC). Eachsignal 10A-10E is then applied to processing circuitry that includes:(1) a return path for allowing for some of the current flowing from theignition control board to flow through the processing circuit and backto the source of the signal; and (2) a active path that, depending onthe state of the input signals 10, will pass through one or moreoptocouplers to set the states of the three logic level output signals12. The optocouplers are configured to provide outputs signals at logiclevels suitable for processing by the digital controller 18.

[0046] Each of the return paths for each of the five high voltage levelsignals 10A-10E includes an initial input resistor (30 a for signals10A, 30 b for signals 10B, etc.), coupled in series with a parallelconnection of a resistor 31 a-31 e and capacitor 32 a-32 e coupled to acommon return path. In the illustrate embodiment of FIG. 5, two commonreturn paths are provided such that the same circuit 8 can handle inputsignals at 24V or 115VAC levels. A first return 33 is provided forhandling 24V control signals. A second return 34 is provided thatincludes a drop-down resistor 35 that may be used when an ignitioncontrol provides very high output voltage signals at, for example, a115VAC level.

[0047] In addition to being provided to the first return path describedabove, the signal from the ignition control board is applied to asecondary processing circuitry that combines and converts the signal tothree digital logic level signals. The secondary processing is slightlydifferent for each signal from the ignition control board and combinesthe five high-level voltage signals 10A-10E to produce three logic levelcontrol signals 12A-12C.

[0048] It should be understood that the precise nature of the secondaryprocessing circuitry may vary depending on the precise form that theinput signals 10 from the thermostat or ignition control board take. Ingeneral, because the input conversion circuitry 8 provides three logiclevel output signals 12A-12C, there are eight possible operating statesof the system. In the exemplary embodiment described herein, however,only five of these states are utilized with the five utilized statescorresponding to: HI COOL, LOW COOL, HIGH HEAT, LOW HEAT ORFAN/RECIRCULATION. In general, the nature of the secondary processingcircuitry is such that the input signals 10 produce the combination ofthe logic level signals 12 that corresponds to the operating modecommanded by the thermostat or furnace board. For example, if thecombination of logic level signals 111 corresponds to FAN/RECIRCULATIONthe second circuitry should be configured such that the receipt of the24V or 115VAC signal(s) corresponding to the circulate mode wouldproduce the desired logic level output signal 111.

[0049] In the exemplary embodiment of FIG. 5, the three logic levelsignals 12A-12C are provided, respectively, as outputs from threeoptocouplers 35A-35C. The optocouplers 35A-35C provide a mechanism forconverting the high-level voltage signals to logic level signals and forisolating the high voltage side of the input conversion circuit 8 fromthe logic level side of the circuit, thus providing some degree ofintrinsic safety.

[0050] Each of the optocouplers 35A-35C has, on the input side, twoinput terminals and, on the output side, two output terminals. In FIG.5, the upper output terminals of the optocouplers 35A-35C are tied tothe logic supply voltage Vcc. The lower output terminals of theoptocouplers 35A-35C provide the logic level signals 12A-12C. Such loweroutput terminals are all coupled to a common ground point 36 throughparallel connections 37A-37C of a resistor and a capacitor. Theseresistor-capacitor networks thus normally provide logic low levels onthe signals 12A-12C when the optocouplers are off. However, when one ofthe optocouplers 35A-35C is turned on, it will pull its associatedoutput signal to the high logic state.

[0051] The value of the logic level output signals from the optocouplers35A-35C will be determined by the value of the input signals 10A-10E. Inthe exemplary embodiment of FIG. 3, the logic level signal 12A will bedetermined by the input to the upper input terminal of optocoupler 35Awhich corresponds directly to the input signal 10A. Thus, whenever thelevel of the 10A signals is at a high level, current will flow frominput 10A, through a diode 38A, through optocoupler 35A and throughzener diode 39 to one of the two return paths (33 or 34). The zenerdiode 39 should be selected to control the voltage threshold for the24VAC inputs and the resistor 305 should be selected to control theamount of current flowing through the optocoupler with 115VAC inputs toensure that the optocoupler 35A is not damaged or overloaded. Thecurrent signal flowing through the input terminals of the optocoupler35A will be controlled by the precise nature of the secondary circuitryand, as described above, should properly map the input signals 10A-10Eto the appropriate logic level signals 12A-12C.

[0052] While the above discussion focuses on the impact of signal 10A onoutput 12A, the impact of the other input signals 10A-10E on the logiclevel output signals 12A-12C would be apparent to one of ordinary skillin the art having the benefit of this disclosure.

[0053] Thus, in the manner described above, the input conversion circuit8 can convert five conventional high level voltage signals from theignition control board are converted into three logic level digitalsignals for application to the microprocessor-based controller 18.

[0054] As described above, the microprocessor-based controller 18receives the logic control signals 12A-12C and in response to thesesignals—and other signals as described below—controls the switching ofthe inverter to operate the PSC motor 2 in one of several possibleoperating modes.

[0055] Referring to FIGS. 3A and 3B, the microprocessor-based controller18 may be any suitable controller such as, for example, the MCUMC68HC908JK3 available from Motorola. The controller should include aninterface for receiving the logic level control signals 12A-12C as wellas the field adjustment signals 16, to be discussed in more detailbelow.

[0056] In general, the logic level signals 12A-12C determine theoperating state of the microcontroller 18. As described above, the logiclevel signals that define the operating state may come from the inputconversion circuit 8 or directly from a thermostat designed to providesuch logic level outputs. Such a thermostat may use, for example, serialcommunication through the optocouplers for isolation or an RFcommunications link.

[0057] In addition to being able to provide control capability tomultiple operating states, the controller 18 of the present disclosuremay be programmed to operate in one or more operating modes. Forexample, the controller may be configured to operate in a CURRENTCONTROL MODE, where each operating state in such mode corresponds to adesired motor current. Alternately, the controller may be configured tooperate in a FREQUENCY CONTROL MODE, where each operating state in suchmode defines a desired output voltage frequency. Still further, thecontroller 18 may be configured to operate in a SPEED CONTROL MODE wherethe output speed of the motor is controlled or a POWER CONTROL MODEwhere the power output of the inverter coupled to the PSC motor iscontrolled. Still further embodiments are envisioned where, depending onthe types of inputs received by the controller 18, the controller may beconfigured to switch among any of the described—or otherpossible—operating modes.

[0058] The operating of the controller 18 in the CURRENT CONTROL MODEwill be initially discussed.

[0059] In the CURRENT CONTROL MODE, each of the eight possible operatingstates (as defined by the logic level inputs 12A-12C) will correspond toa desired current level in the PSC motor 2. In this mode, a currentfeedback signal will be provided to the controller 18 to provide anindication to the controller of the magnitude of the current in themotor winding. The current feedback signal may be obtained from acurrent sensor coupled to the windings of the PSC motor 2 or derivedfrom a current sensor or sensor positioned within one or both legs ofthe inverter 4.

[0060] In one exemplary embodiment, the current feedback signal providedto the controller 18 is taken from a current sensor in the lower leg ofthe DC bus in the inverter 4. This embodiment is illustrated generallyin FIG. 3B and FIG. 6.

[0061] Referring to FIG. 3B, it may be noted that there exists a shuntresistor 60 that is positioned in the lower leg of the DC bus. A voltagereading from this shunt resistor is provided as an input to a lower legcurrent monitoring circuit, that is illustrated in more detail in FIG.6. Because the voltage across the shunt resistor 60 will vary withchanges in the current flowing in the lower leg of the inverter andbecause the current flowing in the lower leg of the inverter willcorrespond to the current in the PSC motor 2, the voltage from the shuntresistor 60 provides an indication of the current in the PSC motor 2.

[0062] Referring to FIG. 6, the voltage from the shunt resistor 60 isprovided as an input to two differential amplifiers 63 and 64.Differential amplifier 63 is configured as a comparator and it comparesthe detected voltage value to a reference value and generates a lowercurrent trip signal on line 65 in the event that the voltage valueexceeds a predetermined value. As described in more detail below, thelower current trip signal may result in a resetting of the controller18.

[0063] As reflected in FIG. 6, the voltage from the shunt resistor 60 isalso applied as an input to differential amplifier 64. Differentialamplifier 64 is configured to perform some filtering and voltage leveladjustment of the signal from the shunt resistor to product an outputvoltage signal on line 66 that varies with, and corresponds to, thevoltage from the shunt resistor 60 and, therefore, that varies with andcorresponds to the current flowing in the PSC motor 2. Differentialamplifier 64 should be configured to produce an output voltage thatvaries in response to the input voltage but where the maximum expectedoutput voltage on line 66 will be less than the maximum input voltage ofthe A to D converter and the logic supply voltage supplied to thecontroller 18.

[0064] While FIGS. 3B and 6 illustrate the use of a shunt resistor togenerate a signal representative of the PSC motor current, other formsof current detection may be used.

[0065] In the current control mode, the microcontroller 18 will comparethe value of the current feedback signal with the desired current levelfor the selected operating state. If the comparison indicates that themotor current is less than the desired setpoint current, then thecontroller 18 will increase the output voltage and frequency applied tothe windings of motor 2 so as to tend to increase the current in themotor 2 by increasing the speed of the blower motor. If the comparisonindicates that the motor current is above the desired setpoint current,then the controller will decrease the voltage and frequency of theoutput voltage to tend to cause the current in the motor to decrease byreducing the speed of the blower motor. This comparison and adjustmentof the output voltage and frequency will regularly occur in an effort tomaintain the current in the motor at the desired setpoint level. Thecomparison and adjustment may be done in software, hardware or firmwareand the implementation of such functionality will be within the level ofone of ordinary skill in the art having the benefit of this disclosure.

[0066] In one embodiment, the relationship between the output voltageand the output frequency will vary depending on the specific operatingstate of the system. In this embodiment, each operating state—inaddition to defining a particular desired current setpoint—will alsodefine a desired volts/hertz curve such that the relationship betweenthe output voltage and the current may vary from operating state tooperating state. In such an embodiment, the volts/hertz curve may takeany appropriate form. In one desired embodiment, linear volts/hertzcurves are used.

[0067]FIG. 7 generally illustrates representative volts/hertz curves forsix exemplary operating states A, B, C, D, E and F. Note that thedesired current setpoints for such operating states are not reflected inFIG. 7.

[0068] Referring to FIG. 7, it may be noted that each of the volts/hertzcurves is linear in that the rate of change of the output frequency isconstant when compared to the rate of change of the output voltage. Inthe illustrated example, each volts/hertz curve also has a minimumoutput frequency and a maximum output frequency. In one embodiment, theminimum output frequency during normal operation of the control systemis 26 Hz and the maximum output frequency is 57 Hz (corresponding to aspeed range for the PSC motor 2 of 500-1100 RPMs). Alternate embodimentsare envisioned wherein different ranges of output frequency arepossible, including embodiments wherein the maximum output frequencyduring normal operation is 60 Hz (the typical line frequency) or evenhigher. In such embodiments when the desired output frequency is at ornear 60 Hz, the controller 18 may be programmed to generate a controlsignal to switch a relay to cause the motor to operate off line power.

[0069] In addition to having minimum and maximum operating frequencies,the voltz/hertz curves of FIG. 7 also define minimum and maximum voltagevalues. Notable, while the minimum and maximum frequency values areshared by the curves for the different operating states, the minimum andmaximum voltage levels may be different. In the example of FIG. 7 eachoperating state defines a different minimum voltage value and differentmaximum voltage value.

[0070] The precise nature of the volts/hertz curves for the variousoperating states should be set to maximize a desired operatingcharacteristic of the system such as, for example, efficiency, noise,vibration, etc. In the embodiment illustrated in FIG. 7, the volts/hertzcurves were selected to provide for maximum operating efficiency.

[0071] This use of differing volts/hertz curves for each operating statein the CURRENT CONTROL mode produces PSC motor tap-like performance, inthat, the energization characteristics of the motor at the differentoperating states causes the motor to operate differently.

[0072] Unlike a tapped PSC motor, however, the use of the CURRENTCONTROL mode as described herein allows for operational advantages thatare not obtainable with a conventional PSC motor and control system. Forexample, if the volts/hertz curves are selected to control the slip ofthe motor, the present system can provide for highly efficientoperation, even at low operating speeds, provided that the volts/hertzcurves are selected to maintain a slip within, for example, the range of100-200 RPM for all of the operating states and for all staticpressures. Alternate embodiments are envisioned where the slip is evenless. Typically the slip will be at least 50 to 75 RPM for aconventional six pole PSC motor. Additionally, because the operatingcharacteristics of the PSC motor 2 are adjusted depending on theoperating state of the system and because it is the current in the motor2 that is being controlled, excess airflow at high speeds and low staticpressures can be eliminated.

[0073]FIG. 8 provides a general comparison of the performance of thecurrent system operating in the current control mode with theperformance of a conventional, multi-tapped PSC motor. Specifically,FIG. 8 illustrates CFM/Static Pressure curves for a system as describedherein operating in the CURRENT MODE and a conventional multi-tapped PSCmotor. The CFM/Static Pressure curves for the system of the presentinvention are illustrated in bold for six different operating states andCFM/Static Pressure curves for the conventional PSC motor areillustrated in the light lines for four different taps. As the figureillustrates, for all of the illustrated operating states or tapsettings: (1) the curves associated with the system described herein arestraighter (meaning that the CFM output of the system is more constant);and (2) the system described herein allows for airflows at a lower CFMlevel than is available with the tapped PSC system. Moreover, althoughnot reflected directly in FIG. 8, the system described herein uses lessenergy for the same airflow for all operating states/taps except for thehighest speed tap.

[0074] In addition to being capable of operating in CURRENT CONTROLmode, the controller 18 of the present disclosure can operate in aFREQUENCY CONTROL mode. In the FREQUENCY CONTROL mode, each operatingstate (as defined by the input signals 12A-12C) defines a desired outputvoltage operating frequency. Each output operating frequency will alsocorrespond to a desired output voltage, with the voltage varyinglinearly with changes in the desired output frequency. Thus, in thisFREQUENCY CONTROL mode, the input signals 12A-12C will define a desiredoperating frequency which will have a corresponding desired outputvoltage. The controller will then drive the inverter to provide thedesired output frequency and voltage and the motor current will not bedirectly controlled.

[0075] In the FREQUENCY CONTROL mode, the frequency output for theinverter will correspond roughly to the rotational speed of the motorand, thus, roughly to the blower output. In one embodiment, thecontroller 18 may be configured to drive the inverter to produce one ofeight possible output frequencies. For example, the controller may beconfigured to provide output operating frequencies of 60 Hz, 55 Hz, 50Hz, 45 Hz, 40 Hz, 35 Hz, 30 Hz and 25 Hz with the higher frequencyoutput corresponding to higher blower speeds and generally higher CFMoutputs and the lower frequency outputs corresponding to generally lowerspeeds and lower CFM outputs.

[0076] One potential issue with operating the system in the FREQUENCYCONTROL mode is that the output parameter of most consequence to theuser of the HVAC system in which the motor system is used is notinverter output frequency but rather the CFM moved by the blower. Ingeneral—at a given static pressure—the CFM moved by the blower willcorrespond to the rotational speed of the blower motor, which willcorrespond to the frequency of the inverter output voltage. However, fora given output frequency, the actual CFM moved by the motor will varysignificantly depending on the static pressure against which the bloweris working. Thus, in the FREQUENCY CONTROL mode, the CFM produced from aHIGH HEAT setting will vary depending on the static pressure of thesystem which can be affected by, for example, the ambient atmosphericpressure, the number of doors in a house that are opened or closed, theposition of the return ducts, etc. As such, controlling the inverter toproduce a set frequency does not necessarily result in good CFM control.

[0077] To overcome some of the limitations of the FREQUENCY CONTROLmode, a POWER CONTROL mode may be provided in which each operating statecorresponds to a desired POWER OUTPUT of the inverter. Because theactual work done by the blower will generally correspond to the CFMmoved by the blower—regardless of the static pressure—this form ofcontrol may more accurately control the CFM and provide enhanced controlof the system. Accordingly, under this control scheme, while the outputvoltage magnitude and frequency of the inverter may vary for a givenoperating mode (e.g., HIGH HEAT), the actual CFM for the mode will berelatively constant irrespective of changes in the static pressure.

[0078] The work output of the motor can be accomplished by sensing thevoltage applied to the motor and the current drawn by the motor, whichwill indicate the power applied to the motor. Once the power actuallybeing drawn by the motor is detected, the inverter can be controlled toadjust the voltage and/or frequency output of the inverter until thedesired power is being drawn by the motor, and therefore, the desiredamount of work and CFM circulation is being done by the motor. Undersuch a control scheme, the setpoints for the various operating modeswould correspond to desired workloads (or even desired CFM outputs).

[0079] The various operating modes described above may be implementedthrough software, hardware and/or firmware within the controller 18 oran external memory may be provided to determine the functionality of thecontroller 18 and, therefore, the functionality of the system. In oneexemplary embodiment, the software that determines the functionality ofthe controller 18 and, thus, the system, is stored in flash memorylocated within the controller 18. In such an embodiment, a data exchangeport may be provided to allow for updating and modification of thesoftware within the controller 18 and for changing the operating mode ofthe controller. In some embodiments, the data exchange port may also beused for monitoring the operation of the controller 18 and receivingdiagnostic data about the over system.

[0080] As described above, the field adjustment circuit 14 allows forfield adjustment of the setpoints that correspond to the operatingstates defined by the control signals 12A-12C. In general, for eachpossible operating state, some form of circuitry may be provided in thefield adjustment circuit 14 to allow for modification or adjustment ofthe set point corresponding to that operating state. Thus, if thecontroller 18 is operating in the CURRENT CONTROL MODE and the inputsignals can define five valid operating states, with each operatingstate corresponding to a specific current setpoint, the field adjustmentcircuit may allow for modification of the current set pointscorresponding to the various operating states. If the controller 18 isoperating in the FREQUENCY CONTROL MODE, then the field adjustmentcircuitry will allow for adjustment of the frequency setpointscorresponding to the various operating states.

[0081] Because the controller 18 will, in certain embodiments, be adigital controller, the field adjustment circuitry may take the form ofa digital communications interface that would allow an installer,technician or user to couple a digital communications device (e.g., alaptop computer) to the interface. This embodiment, however, requiresthat the installer, technician or user have access to relativelysophisticated equipment and an understanding of how to use suchequipment. Accordingly, for some applications a lower cost, simplerapproach is desirable where few—if any—tools will be required on thepart of the installer, technician or user to provide field adjustment ofthe setpoints corresponding to the operating states.

[0082] One embodiment for illustrating such an elegant, essentiallytooless, approach for providing field adjustability of the setpoints isillustrated in FIG. 3B and FIG. 9A. In the exemplary embodiment of FIG.3B, there are five possible operating states. Accordingly, there arefive dedicated field adjustment circuits 91, 92, 93, 94 and 95, one foreach of the possible operating states. As described above, eachoperating state may correspond to a specific setpoint which—depending onthe operation mode of the controller 18—can be a current setpoint, afrequency setpoint, a speed setpoint or a CFM setpoint. For purposes ofthe present discussion, it will be assumed that the controller isoperating in FREQUENCY CONTROL MODE although it will be appreciated thatthe setpoints could, for example, refer to a desired current setpoint ofthe controller 18 operating in the CURRENT CONTROL MODE.

[0083] Referring to FIG. 3B, each of the field adjustment circuits 91,92, 93, 94 and 95 comprises a string of series connected resistorscoupled across a defined voltage and a set of jumpers that include tapscoupled at various points in the resistor chain. FIG. 9A illustrates ingreater detail one of the field adjustment circuits.

[0084] Referring to FIG. 9A, the exemplary field adjustment circuitincludes three series connected resistors 96, 97 and 98 coupled across a5V bus. The voltage level at one point of resistor 98 is output on line99 as the output voltage of the field adjustment circuit. The pointswhere the resistors are coupled together are provided as inputs to ajumper box 100 that provides, in the illustrated example, five accesspoints to which jumpers may be coupled.

[0085] As those of ordinary skill in the art will appreciate, thevoltage level at the output 99 will be dependant on the manner in whichjumpers are positioned within the jumper box 100. Thus, by manipulatingthe placement of jumpers in the jumper box, it is possible to adjust theoutput voltage at point 99 and, thus, provide different field adjustmentsignals.

[0086] For example, if a jumper 101 is positioned to couple Pin #1(which is coupled to ground) to Pin #2 (which is coupled to the output99), the output voltage on the output line will go to ground, thusproviding one level of field adjustment. This is generally illustratedin FIG. 9B. Alternately, if a jumper 102 is positioned to couple Pin #2to Pin # 3 (which is connected to +5 V), the voltage level at point 99will rise to the 5V logic supply level and this will define a secondfield adjustment. This is generally illustrated in FIG. 9C. Stillfurther, if a jumper is provided to couple Pin #5 to Pin #4, 5V will beprovided across resistors 97 and 98 only and the output voltage at point99 will take a third state defining a third field adjustment. Finally,if no jumpers are employed, the voltage at point 99 will correspond tothe voltage at point 99 when 5V is applied across the entire resistorchain 96, 97 and 98 thus defining a fourth field adjustment. Thus,through the relatively simple circuitry of FIGS. 3B and 9A simple,tooless field adjustments can be made to generate one of four fieldadjustment signals.

[0087] The field adjustment signals from the field adjustment circuits91-95 may be processed by the controller 18 in a variety of ways toadjust the setpoints for the various operating state. In one embodiment,the controller may be programmed to sample the value of the fieldadjustment signal for each operating state and, for each sampled value,adjust the initially established setpoint for that operating state up ordown by a predefined amount. Alternate embodiments are envisionedwherein the field adjustment signals are used to select one of a numberof possible predefined setpoints for that operating state. Thisembodiment is beneficial in the respect that it ensures that, regardlessof the type of field adjustments that are made, the controller 18 willbe controlling the system to a known defined setpoint. By ensuring thatthe controller 18 will always control the system to a limited number ofdefined setpoints, it is possible to optimize the system (e.g., bywinding the motor to operate efficiently at the predefined setpoints).It is also possible to reduce the complexity and costs of the controllerbecause the controller will only need to control the system to a limitednumber of defined setpoints.

[0088] Embodiments are envisioned wherein, for each operating state,several unique setpoints are provided. Thus, in the example of FIGS. 3Band 9A, where there are five operating states and each field adjustmentcircuit is capable of providing one of four field adjustment signals,the total number of possible predefined operating states is 5×4 or 20.While such an embodiment may have advantages, it is possible to reducethe complexity of the system by limiting the total number of availablesetpoints to something less than the total number of possible states andhaving some of the potential operating states overlap. One embodiment ofa control scheme that implements such an approach is illustrated in FIG.10.

[0089]FIG. 10 illustrates a control scheme that may be implemented bycontroller 18 when the controller is operating in the FREQUENCY CONTROLMODE, although it will be appreciated that the same control scheme canbe used in the other operating modes.

[0090] Referring to FIG. 10, the leftmost grouping of lines representsthe total number of setpoints at which the controller 18 is programmedto operate. In the illustrated example, the controller 18 has beenprogrammed to operate at eight possible frequency setpoints (60 Hz, 55Hz, 50 Hz, 45 Hz, 40 Hz, 35 Hz, 30 Hz, and 25 Hz) Thus, the controller18 and the remaining circuitry of the system 1—including the PSC motor2—can be optimized to run at only these eight frequencies. The groupingof lines to the right represent the possible setpoints for the availableoperating states. In the illustrated example, there are five possibleoperating states (COOL 1, COOL 2, HEAT 1, HEAT 2 and FAN). Each of thefive operating states has associated with it four possible frequencysetpoints. For example, the COOL 1 operating state corresponds topossible frequency setpoints of 60 Hz, 55 Hz, 50 Hz, and 45 Hz. The FANoperating state, in turn, corresponds to possible frequency setpoints of60 Hz, 40 Hz, 30 Hz, and 25 Hz.

[0091] In the illustrated example, each of the four possible frequencysetpoints will correspond to one of the possible field adjustment signalvalues. Thus, if the logic level control signals indicate that thecontroller 18 is to be in the FAN operating state, the controller 18will sample the field adjustment signal corresponding to the FANoperating state. Depending on which of the four possible values thatfield adjustment signals takes, the controller 18 will select one of thefour available setpoints and control the output of the inverter to thatsetpoint. The same control function would occur if a different operatingstate was selected.

[0092] While the above has been described in terms of controller 18operating in the FREQUENCY MODE, the same control scheme could be usedin different modes. In such different modes, the predefined setpointscould correspond, for example, to current levels, CFM levels, or powerlevels. In any of these cases, if the approach of FIG. 10 is used, thesystem can be optimized to run at the limited number of defined setpoints.

[0093] While the above discussion discusses field adjustment in thecontext of adjusting the setpoint values, other forms of fieldadjustment are envisioned. For example, it may be possible to develop asingle controller 18 that can drive both 1 Hp and ½ Hp motors. In suchan embodiment, it may be desirable to provide a motor selection circuitas provided to allow the installer of a HVAC system as describe hereinto select the type of motor that will be coupled to the system. Forexample, if the controller 18 can work with ½ Hp and 1 Hp motors, ajumper may be provided that, depending on the state of the jumper, willallow the controller to optimally control either a ½ Hp or a 1 Hp motor.Alternate embodiments are envisioned wherein switching elements otherthan jumpers (e.g., DIP switches) are used to allow for fieldconfiguration for a larger number of motors. Still further fieldadjustment circuits are envisioned for allowing an installer, technicianor user to set the operating mode of the controller 18.

[0094] In addition to controlling the motor 2 during normal operation tooperate in a manner consistent with the then-current setpoint (asdefined by the input control signals and as adjusted by the fieldadjustment signals), the controller 18 can also implement specializedcontrol routines during start-up of the motor (i.e., when the motor goesfrom an unenergized state to an energized state).

[0095] There are several beneficial methods that the controller 18 mayimplement to start the PSC motor 2 from a stopped or standstill state.

[0096] In one exemplary approach, illustrated generally in FIG. 11,during a start-up operation where the PSC motor 2 goes from an off stateto a running state, the voltage and frequency output of the inverterwill be controlled from a predetermined frequency to provide optimumstarting of the motor. In this embodiment, upon the detection of astartup operation (i.e., upon the detection that the motor is beingstarted from a stopped or standstill state), the controller will providean output voltage that initially ramps up very rapidly (region 110)(almost instantaneously) in a linear manner from zero volts and zerohertz to a magnitude corresponding to between 30%-70% of the availablebus voltage (in one embodiment approximately half of the available busvoltage) and a frequency equal to approximately 30 Hz (approximatelyhalf of the line frequency). Notably, at the 30 Hz point duringstart-up, the output voltage (half of the available bus voltage(50-60V)) will be higher than the output voltage that would correspondto a 30 Hz output frequency during normal operation. Once the outputfrequency reaches the approximately 30 Hz point, the frequency andmagnitude of the inverter output voltage are, in this approach,maintained constant for a defined period of time (e.g., 5seconds)(region 112). After remaining at the start-up voltage magnitudecorresponding to 30 Hz for the defined period of time, in whichapproach, the controller 18 will cause the voltage magnitude to drop tothe normal operating voltage at 30 Hz for the operating state underwhich the controller is operating (region 113) and then adjust theoutput of the inverter to reach the desired frequency output and thevoltage magnitude corresponding to the then present operating state isin accordance with the volts/hertz curve for that operating state. Thisis reflected by the dotted line in FIG. 11 (region 114) where (in theexample) the ultimate inverter output is near the 75% of maximum voltageand the output frequency at that voltage level is near 45 Hz. Notably,while the voltage frequency and magnitude have a linear relationshipduring the normal operating mode, the rate of change of the voltage overthe rate of change of the frequency is significantly lower than for thestart-up mode.

[0097] It is believed that the fast increase in the voltage duringstart-up to a relatively high “start-up” value at a selected start-upfrequency (e.g., 30 Hz), and the maintenance of the voltage at thisstart-up value and frequency for a predetermined period of time and anadjustment of the voltage to a value for the start-up frequency that isless than the start-up voltage and thereafter varying thevoltage/frequency in a linear manner, softly starts the motor in amanner that is safe, that does not put undue strain on the motor or theinverter, and that is quiet.

[0098] While the start-up approach described above is believed toprovide certain benefits, for certain applications the starting torqueavailable from that approach is insufficient to start the motor 2 in adesirable manner. In such applications, an alternate starting approachis often desirable.

[0099] Because the amount of starting torque will vary with the amountof current flowing through the auxiliary windings of the PSC motor 2(i.e., the winding that is coupled in series with the capacitor), thealternate approach controls the energization of the motor to place alarger share of the current in the auxiliary windings upon start-up and,therefore, produce a relatively large amount of starting torque. This isaccomplished by controlling the frequency of the voltage applied to themotor during start-up. As those of ordinary skill in the art willappreciate, the impendence of a capacitor is less for a high frequencyvoltage signal than for a lower frequency voltage signal. Thus, byincreasing the frequency of the applied voltage during start-up, it ispossible to decrease the apparent impedance of the auxiliary windingand, therefore, increase the current flowing in the auxiliary windingand, therefore, the starting torque.

[0100] An exemplary start-up process in which the frequency output ofthe inverter is driven to above line frequency during start-up isillustrated generally in FIG. 12.

[0101] Referring to FIG. 12, a start-up process is illustrated wherein,during start-up, the controller 18 will rapidly bring the inverteroutput to a voltage magnitude level that is somewhere betweenapproximately 30% and 70% of the available voltage (for example—in oneembodiment—to a near half voltage level, 50-60V) but to a relativelyhigh frequency value that is above the line frequency value such as, forexample, 74 Hz (region 120). The output for the inverter will remain atthis relatively high frequency and at the half voltage level for apredetermined period of time (e.g., five seconds)(region 122) and will,thereafter, go to approximately one-half line frequency (e.g. to about36 Hz.) and then immediately ramp in a linear manner to the outputdefined by the current operating state and mode of the controller 18.The inverter output is brought to approximately half line frequency (36Hz.) in this process because, during startup the motor is still comingup to speed and allowing the motor to lock-in at approximately 2 linefrequency is believed to be beneficial.

[0102] In the embodiment of FIG. 12, the controller 18 will monitor thecurrent flowing through the lower leg of the inverter using thecircuitry described above (or some other signal that will indicatewhether the motor is turning) and if the monitoring indicates that themotor was not properly started it will then initiate a secondary startoperation where the output of the inverter is rapidly brought back tothe approximately ½ voltage and 74 Hz output level and maintained atthat level for longer period of time than it was during the initialstarting operation (e.g., for a period of 20 seconds). The controller 18will then ramp the output to the output corresponding to the operatingstate and operating mode of the controller. If the monitoring of thecurrent (or other data reflecting the motor operation) indicates thatthe motor was not properly started, the secondary start operation willbe repeated.

[0103] Through the use of the above-line frequency starting methoddescribed above, faster and better motor starting is believed to bepossible.

[0104] In addition to controlling the operation of the inverter so as toprovide desired control of the PSC motor 2 during start-up and normaloperation, the controller 18 may also be used to control relay 6 toessentially bypass the inverter and couple the input terminals of thePSC motor directly to line power. This capability potentially providesfor relatively high efficiency operation at outputs at or near linefrequencies because the losses caused by the inverter (e.g., switchinglosses, etc.) are not incurred when the motor 2 is running directly offthe line.

[0105] In the example of FIG. 3B, the controller 18 determines the stateof the relay 6 through the utilization of a switching circuit 130 in theform of a power transistor that is coupled in series with the relaywinding across a voltage supply. The gate drive of the transistor iscoupled to an output of the controller 18 such that by changing thestate of the relevant output, the controller can selectively switchrelay 6 to couple the PSC motor to the inverter 4 or to the line.

[0106] In one exemplary embodiment, the controller 18 may be programmedto switch the relay to couple the motor 2 to the line whenever thedesired output operating frequency during normal operation meets orexceeds a desired maximum frequency value (e.g., switch to line when thedesired output frequency is between 57 Hz and 62 Hz). Alternately, thecontroller may be programmed to switch to the line only when aparticular sequence of the input control signals 12A-12C is detected(e.g., 111). Still further embodiments are envisioned where thecontroller 18 will switch the relay to drive the motor from the linewhenever the frequency is within a pre-defined range or a particularsequence of input command signals 12A-12C is detected.

[0107] In one embodiment, whenever the controller 18 is to transfer themotor 2 from the inverter output to the line, the controller 18 willramp the voltage and frequency output of the inverter to the maximumpossible voltage output and maximum frequency and then turn off theenergization of the inverter without transferring the motor 2 to theline. The inverter will then remain off, with the relay coupling themotor to the inverter such that the motor remains de-energized for adefined period of time such as, for example, 0.5 seconds. After thisdefined period of de-energization, the relay will be switched to couplethe motor to the line. The use of this period of completede-energization is believed to provide for a smooth transfer from theinverter 4 to the line. It assures that the relay does not interrupt orbreak the inverter current. The relay 6 serves to protect the inverterby ensuring a break before make situations such that the inverter isnever coupled to the motor when the motor is coupled to the line.

[0108] While the above procedure describes the process for transitioningenergization of the motor from the inverter to the line, instances willarise where the energization of the motor must be transitioned from theline to the inverter. In accordance with one embodiment, theenergization of the motor is transitioned from the line to the inverterin accordance with a controlled process. In this process, before therelay is switched to transfer the energization of the motor from theline to the inverter, the inverter output is brought to a frequency thatis very near to line frequency (e.g., 58 Hz) before the motor istransferred to the inverter. However, the magnitude of the voltage ofthe inverter is controlled such that the voltage magnitude of theinverter is approximately half of the voltage magnitude that would existat 58 Hz if the motor were being driven by the inverter during normalconditions. When the inverter output is set at a near line frequency(e.g., 58 Hz) and a half-normal voltage magnitude, the motor is thenswitched from the line to the inverter. The frequency of the inverter isthen maintained at 58 Hz and the magnitude of the voltage is rapidlyincreased from the half-normal voltage level to a voltage level thatcorresponds to the voltage output at the selected frequency (58 Hz)during normal operation. The voltage and frequency of the inverter arethen controlled to take the voltage to the desired output frequency andthe corresponding voltage.

[0109] It is believed that the reduction in the voltage to half of thenormal operating voltage at the inverter transfer frequency, and thequick increase in the voltage up to normal voltage for the transferfrequency, places less strain on the inverter than would a transfer atfull voltage and provides for a smoother transition of the motor fromline to inverter.

[0110] In addition to using the relay 6 to transition the energizationof the PSC motor 2 from the inverter 4 to the line, certain embodimentsof the present system can use the relay 6 to preclude the inverter 4 andthe controller 18 from starting in an unsafe or unstable mode and/orensure that a failure of the controller 18 and/or the inverter 4 wouldnot completely disable the motor 2, but would instead cause the motor torun off of the line voltage such that if the inverter 4 and/orcontroller 18 failed, the motor 2 would continue to run.

[0111] In one embodiment of such a system the relay 6 may be configuredsuch that, in its normal-unenergized state, the PSC motor is coupled tothe line. In this embodiment, the controller 18 can, by energizing therelay, switch the relay 6 such that the motor is connected to the outputof the inverter. In this design, if the controller 18 fails, the relaywould remain in its normal, unenergized state and would couple the motor2 to the line. As such, the failure of the controller 18 would result inthe PSC motor 2 safely operating off line power.

[0112] In an alternate embodiment, the relay 6 may be selected such thatthe relay, in its unenergized state, couples the PSC motor 2 to theinverter 4. One example of such an embodiment is illustrated in FIG. 13.

[0113] Referring to FIG. 13, a relay 6 is provided that includes anenergization coil 165 that is coupled on one end to a source of DC powerand on its other end to a switching device 163. The switching device 163is selected such that, in the presence of an adequate gate voltage, thedevice 163 will conduct. Line power is provided to the relay on line 160and the output of the inverter is provided on line 161. The output ofthe relay 162 is coupled to the PSC motor (not illustrated). The relayis configured such that, when the relay coil 165 is unenergized, therelay will couple the inverter output 161 to the motor lead 162. Asecond switching device 164 whose gate is coupled to an output of thecontroller 18 is also provided.

[0114] In the illustrated embodiment, the gate of the switching device163 is coupled to a source of voltage 166 that may be a DC valuecorresponding to the line voltage. Appropriate step down resistors maybe used to provide an arrangement such that, whenever the line voltageis sufficient to drive the motor properly, the transistor 163 will(assuming that device 164 is non-conductive) conduct and energize therelay thus coupling the motor input 162 to the line. Thus, in theabsence of the energization of the switching device 164, if the powersupplied to the system is sufficient to safely drive the motor 2, themotor will be coupled to the line and will run off of the line.

[0115] In the illustrated embodiment, the gate signal for switchingdevice 164 is coupled to an output of the controller 18. Thus, assumingthat the voltage 166 is sufficiently high, the status of device 164 willdetermine the energization source for the motor. If the switching device164 is conductive, the gate of device 163 will be pulled to ground andthe relay coil 165 will be de-energized, thus causing the relay tocouple the motor to the inverter output 161. If, however, the switchingdevice 164 is not conductive, then the voltage from point 166 will causetransistor 163 to conduct, thus energizing the relay and coupling themotor to the line. In the described embodiment, the controller 18 isconfigured such that it will not come on if the power supplied to thecontroller is inadequate to properly operate the controller 18.

[0116] As those of ordinary skill in the art having the benefit of thisdisclosure will appreciate, if the line voltage is adequate to safelydrive the motor, but the controller 18 fails or is not renderedoperable, the relay coil 165 would be energized, thus resulting in asafe failure where the motor operated off the line. If, however, theline power was inadequate to properly energize the relay—in which casethe power would be inadequate to properly energize thecontroller/inverter—the relay would become unenergized and the relaywould switch to couple the motor to the inverter 4. However, because thecontroller 18 would be inoperable if such a low voltage conditionexisted, the inverter 4 would not receive any switching signals and nopower would be applied to the motor. As such, this embodiment providestwo fail-safe modes: (i) a first mode where the controller fails but thepower is adequate to drive the motor where the motor would run off theline; and (ii) a second mode here the controller is inoperable and/orthe power was inadequate to drive the motor where the motor would not beenergized at all.

[0117] In addition to providing for safe failures in the event ofinadequate supply voltage or failure of the controller 18, the system 1of the present disclosure may be configured to protect or reset thecontroller in the event that the logic supply level is not appropriateor that excessive currents are detected in the inverter. Such protectioncircuitry may take the form of the protection circuit 170 illustrated inFIG. 3B.

[0118] Referring to FIG. 3B, protection circuit 170 comprises aswitching device 171 that is coupled to a reset pin of the controller 18at one terminal and to ground on the other terminal. The device 171 isconfigured such that if the device 171 is rendered conductive, the resetpin of the controller 18 will be pulled to ground, thus resetting thecontroller 18.

[0119] The conductivity of the device 171 is determined by the gatevoltage of the device. The gate voltage of device 171 is, in turn,determined by a variety of input signals. For example, in the embodimentof FIG. 3B the gate of device 171 is coupled to the CURRENT TRIP output65 that monitors the current in the lower leg of the inverter. Thus thedetection of a high current in the lower leg of the inverter will causethe controller 18 to reset. The gate of the switching device 171 is alsocoupled to the output of an upper current trip circuit 172 that isillustrated in more detail in FIG. 14.

[0120] The upper trip circuit 172 is used in the embodiment of FIG. 3Bbecause the inverter 4 of FIG. 3B is based on the use of a voltagedoubler such that positive and negative voltage rails are provided. Whenthe inverter 4 is switched such that the negative rail of the inverteris powering the motor 2, the current can be detected using the circuitryof FIG. 6. When the positive rail of the inverter is powering the motor,however, the circuitry of FIG. 6 will not detect the current actuallyflowing to the motor 2. Accordingly, in the embodiment of FIG. 3Badditional circuitry 172 is provided to detect the excessive current inthe upper leg of the inverter.

[0121] Referring to FIG. 14 the upper current detection circuitry 172comprises a switching device 173 and a shunt resistor 174. The shuntresistor is coupled across one terminal and the gate of the switchingdevice 173. Accordingly, the switching device 173 will be renderedconductive whenever the voltage across the shunt resistor 174 exceeds apredetermined value. Since the voltage across the resistor 174 willcorrespond to the current flowing through the resistor 174, theswitching device 173 will, thus, be rendered conductive whenever thecurrent in the upper leg of the inverter exceeds a predefined value. Theupper trip circuit also includes an optocoupler 176 having an output176. Whenever the transistor 173 is rendered conductive, the optocoupler175 will conduct and a pulse will be generated at the output 176 of theoptocoupler.

[0122] Referring back to FIG. 3B and the protection circuitry 170, itwill be noted that the output of the optocoupler 176 (UT) is coupled tothe gate of switching device 171. Thus, whenever the upper currentdetection circuit detects an excessive inverter current, transistor 171will conduct and the controller 18 will be reset.

[0123] The gate of switching device 171 is also coupled to the output ofa differential amplifier 177 configured as a comparator that comparesthe 15 volt gate drives supply voltage to the logic supply voltage.Whenever the comparator 177 indicates that the gate driver supplyvoltage is insufficient or below an acceptable level for safe operationof the inverter the switching devices, switching device 171 will berendered conductive, thus resetting the controller 18.

[0124] A still further fail-safe mode is envisioned wherein thecontroller 18 monitors the current from the inverter and, if duringnormal operation where current should be flowing to the motor, thecontroller 18 detects that either no current or very little current isgoing to the motor but the control inputs are calling for energizationof the motor, the controller 18 would generate control signals tooperate the relay 6 to connect the motor to the line power. In oneexemplary embodiment, the relay 6 is switched by the controller 18 tocouple the motor to line if the output current of the inverter remainsbelow a minimum value for a period of time of between 1 to 5 secondsdespite the fact that the inputs are calling for an operating statewhere some current is to be provided to the motor. This failsafe modecould allow continued operation of the motor in circumstances where afailed driver IC or other component failure would otherwise keep therelay 6 in a state coupling the motor to the inverter, but where theinverter could not provide the output current when it is required.

[0125] Once the controller 18 has initiated appropriate start-up of themotor, and assuming that no faults are detected such that the controller18 is reset, the controller 18 will determine a desired output voltageand frequency as described above and will produce a pulse widthmodulated (PWM) output having a duty cycle that—when applied throughdriver circuitry in the inverter to the inverter switching devices—willproduce a synthesized sinusoidal voltage signal at the output of theinverter.

[0126] Such switching may be accomplished by having an eight-bit lookupsine table with, for example, 256 stored points stored in a ROM withinor accessible by he controller. The synthesized sinusoidal AC voltageoutput at the inverter may be established by changing the PWM dutycycles of the switching signals to have the general shape of a sinewave. The PWM duty cycle for any given point will be based on the valueof the sine wave at that point. Thus, a 100% duty cycle (the maximumvoltage) will correspond to the peak of the sine wave while a 50% dutycycle will correspond to the zero crossing of the sine wave and a 0%duty cycle will corresponds to the negative peak of the sine wave.

[0127] The inverter 4 may take the form of an inverter based on avoltage doubler—such as the inverter 4 of FIG. 3B—where individualswitching devices are provided for generating the positive and negativeportions of the output sine wave. Appropriate driver circuitry may beprovided to convert the PWM signal from the controller 18 to drivesignals for the voltage doubled inverter.

[0128] Alternate embodiments are envisioned wherein the inverter doesnot use a voltage doubler, but instead uses a single DC buss obtained byfull wave rectifying the input line voltage. The input line voltage maybe, for example, 115 VAC or 230 VAC. Such an embodiment is schematicallyillustrated in FIG. 15.

[0129] Referring to FIG. 15, a controller 18 provides PWM drive signals(generated as described above) to drive circuits 181 and 182 which, inturn, drive sets of power switching devices. The power switching devices(which may be power switching devices such as IGBT's) are coupled toprovide the DC buss of the inverter across the terminals of the PSCmotor so as to generate positive or negative voltages. The use of suchinverter circuitry is known in the art and will not be further discussedherein. As those of ordinary skill in the art will appreciate suchswitching devices will include or require free wheeling diodes.

[0130] The exemplary motor control system described herein may take theform of a separately mounted control module (that will include theinverter and control circuitry) and a PSC motor. The motor may bemounted inside a blower wheel and the controller may be mountedelsewhere in the blower cabinet, preferably in a location where it canutilize some of the airflow from the blower to minimize the rise of theheat sink temperature of the controller. The motor leads should beadequately sized to directly connect to the output terminals of thecontrol module. The controller may be energized by directly connectingthe controller module power input to normal household 115 VAC or to 230VAC power depending on whether the end application is a furnace or aconventional air handler.

[0131] In one exemplary embodiment, the control module of the system 1(which includes all major components of the system except for the motor)is mounted within a bracket like device that may be readily secured to ablower enclosure. Such an embodiment is generally illustrated in FIG.16A where the control module 190 is positioned within a generallyL-shaped bracket assembly that is hingedly coupled to a lid element 192.The lid element 192, in turn, is mounted to the curved exterior of ablower housing by, for example, screws.

[0132] In the embodiment of FIG. 16A, the bracket 191 is coupled to thelid 192 through the use of slot openings in the bracket 191 and hooks193 in the lid 192. Details of one such hook are provided in FIG. 16B.In general, the hooks 193 are passed through the bracket 191 such thatthe bracket 191 may be maintained in two positions. In a closed positionthe bracket 191 is affixed to the lid by a screw or bolt 194. In thisposition access to the control board 190 is precluded. In the otherposition, when screw 194 is removed, the bracket 191 can “swing open” tothe position corresponding to the dotted lines of FIG. 16A thusproviding access to the board 190.

[0133] In alternate embodiments, the hooks 193 may be replaced by tabs.Such an alternate embodiment is illustrated generally in FIG. 16C.

[0134] The use of the mounting structure of FIGS. 19A and 19B isbelieved to provide a compact, elegant mounting structure that is notprone to vibrations that could produce unwanted noise.

[0135] While the apparatus and methods of this invention have beendescribed in terms of preferred embodiments, it will be apparent tothose skilled in the art that variations may be applied to the processdescribed herein without departing from the concept and scope of theinvention. All such similar substitutes and modifications apparent tothose skilled in the art are deemed to be within the scope and conceptof the invention.

What is claimed is:
 1. A field-adjustable motor control systemcomprising: a motor; an inverter coupled to provide energization to themotor; a controller coupled to the inverter, the controller providingsignals to control the output of the inverter in response to receivedinput control signals and in response to field adjustment signals,wherein the input control signals can define a first and a secondoperating state of the controller, each of the first and secondoperating states corresponding to a desired operating state of thesystem; a first field adjustment circuit for providing a first fieldadjustment signal to the controller, the first field adjustment signal,in combination with the input control signals, defining a desired outputparameter of the inverter for the first operating state; and a relaycoupled at its inputs to a source of line power and to the output of theinverter and at its output to the motor, the relay receiving controlsignals from the controller and being configured such that, in the eventof a failure of the controller, the relay will couple the line power tothe motor.
 2. The field adjustable motor control system of claim 1wherein: the controller is a digital controller; the controller isconfigured such that the first operating state is associated with aplurality of potential control set points for the inverter; and thefirst field adjustment signal determines the particular control setpoint to which the inverter is controlled in response to thecontroller's receipt of input control signals corresponding to the firstoperating state.
 3. The field adjustable motor control system of claim 2wherein the plurality of control set points corresponds to desiredoutput current levels.
 4. The field adjustable motor control system ofclaim 1 further comprising: a second field adjustment circuit forproviding a second field adjustment signal to the controller, the secondfield adjustment signal, in combination with the input control signals,defining a desired output parameter of the inverter for the secondoperating state; and wherein the controller is a digital controller; thecontroller is configured such that the first operating state isassociated with a plurality of potential control set points for theinverter and the second operating state is associated with a pluralityof potential control set points for the inverter; the first fieldadjustment signal determines the particular control set point to whichthe inverter is controlled in response to the controllers receipt ofinput control signals corresponding to the first operating state and thesecond field adjustment signal determines the particular control setpoint to which the inverter is controlled in response to thecontroller's receipt of input control signals corresponding to thesecond operating state; and wherein there is at least some overlap ofthe potential set points associated with the first operating state andthe potential set points associated with the second operating state. 5.The field adjustable motor control system of claim 1 wherein the firstfield adjustment circuit may be modified in the field to provide one ormore potential first field adjustment signals.
 6. The field adjustablemotor control system of claim 5 wherein the field modification isaccomplished by the positioning of one or more jumpers within the firstfield adjustment circuit.
 7. The field adjustable motor control systemof claim 1 wherein the relay includes a coil and, when the relay coil isunergized, the relay couples the motor to the source of line power. 8.The field-adjustable motor control system of claim 1 wherein, inresponse to the input control signals defining the first operatingstate, the controller controls the output of the inverter in accordancewith a first volts vs. hertz relationship and wherein, in response tothe input control signals defining the second operating state, thecontroller controls the output of the inverter in accordance with asecond volts vs. hertz relationship, the first volts vs. hertzrelationship being different than the second volts. vs. hertzrelationship.
 9. A field-adjustable motor control system comprising: amotor; an inverter coupled to provide energization to the motor; acontroller coupled to the inverter, the controller providing signals tocontrol the output of the inverter in response to received input controlsignals and in response to field adjustment signals, wherein the inputcontrol signals can define a first and a second operating state of thecontroller, each of the first and second operating states correspondingto a desired operating state of the system; and a first field adjustmentcircuit for providing a first field adjustment signal to the controller,the first field adjustment signal, in combination with the input controlsignals, defining a desired output power level for the inverter for thefirst operating state.
 10. A motor control system comprising: a motor;an inverter coupled to provide energization to the motor; a controllercoupled to the inverter, the controller providing signals to control theoutput of the inverter in response to received input control signals toprovide a level of output power to the motor, the desired level ofoutput power being determined at least in part by the input controlsignals.
 11. The controller of claim 10 wherein the input controlsignals can define a first and a second operating state of thecontroller, each of the first and second operating states correspondingto a desired operating state of the system, further comprising a firstfield adjustment circuit for providing a first field adjustment signalto the controller, the first field adjustment signal, in combinationwith the input control signals, defining a desired output parameter ofthe inverter for the first operating state.
 12. The motor control systemof claim 11 further including: a generally L-shaped bracket assembly; alid element adapted to be coupled to the curved exterior of a blowerhousing, the generally L-shaped bracket assembly being hingedly coupledto the generally L-shaped bracket assembly; and a control modulecontaining circuit components for an inverter and an electroniccontroller, mounted to the generally L-shaped bracket assembly suchthat, when the L-shaped bracket assembly is in a first position whereinthe L-shaped bracket assembly makes contact with the lid assembly,access to the control module is blocked by the bracket assembly, andwhen the L-shaped bracket assembly is swung open along the hingedconnection access to the control module is enabled.
 13. A fail-safemotor control system comprising: a motor; an inverter coupled to provideenergization to the motor; a controller coupled to the inverter, thecontroller providing signals to control the output of the inverter inresponse to received input control signals and in response to fieldadjustment signals, a relay coupled at its inputs to a source of linepower and to the output of the inverter and at its output to the motor,the relay receiving control signals from the controller and beingconfigured such that, in the event of a failure of the controller, therelay will couple the line power to the motor.
 14. The field adjustablemotor control system of claim 13 wherein the relay includes a coil and,when the relay coil is unergized, the relay couples the motor to thesource of line power.
 15. A fail-safe motor control system comprising: amotor; an inverter coupled to provide energization to the motor; acontroller coupled to the inverter, the controller providing signals tocontrol the output of the inverter in response to received input controlsignals, the controller being configured such that it will not providesignals to control the inverter if the power to the controller isinsufficient to ensure proper controller operation; and a relay coupledat its inputs to a source of line power and to the output of theinverter and at its output to the motor, the relay receiving controlsignals from the controller, wherein the relay includes a relay coil,and wherein the relay couples the output of the inverter to the motorwhen the relay coil is in an unergized state; and a switching elementcoupled between the relay coil and a source of power and receiving acontrol signal from the controller, the switching element and the relaycoil being configured such that, in the absence of a control signal fromthe controller, the relay coil is energized and, when a control signalfrom the controller is provided to the switching element, the relay isunergized.
 16. A fail-safe motor control system comprising: a motor; aninverter coupled to provide energization to the motor; a controllercoupled to the inverter, the controller providing signals to control theoutput of the inverter; and means coupled to the motor, the inverter,the controller, and line power for: (a) coupling the motor to line powerwhen the controller fails but line power is sufficient to drive themotor; and (b) coupling the motor to the output of the inverter and notdriving the inverter, such that no power is provided to the motor, whenthe controller fails and line power is inadequate to drive the motor.17. A fail-safe motor control system comprising: a motor; an invertercoupled to provide energization to the motor; a relay coupled at oneinput to a source of line power and at another input to the output ofthe inverter, the relay being coupled at its output to the motor andbeing set in a first state where line power is provided to the motor anda second state where the output of the inverter is provided to themotor; and a controller coupled to the inverter and the relay, thecontroller receiving a signal corresponding to the current output of theinverter, the controller further providing signals to control the outputof the inverter and the state of the relay, wherein the controllermonitors the current from the inverter and, if during normal operationwhere current should be flowing to the motor, the controller detectsthat no current or current below a defined level is going to the motor,the controller provides signals to the place the relay in the firststate.